My 2020 Sierra 3500 dually lugging on easy acceleration

[Kevinsgmc2020]

8070 miles  
So as I take off from a stop sign the truck seems to be lugging  and I guess you’d call it bucking. If I hard accelerate it goes away. It’s only at low rpm. It’s at the dealer now for three days and they tell me they are talking to Gm engineers and truck could be there for a week or two minimum! Obviously after spending 90k$  Not what I want to hear. They’ve never had this problem before. I did need to put it in four wheel drive for a few min to get out of some loose sand on construction site. But put it back in 2 wheel with no problem. Anyone have this problem. Of course I’m also supposed to be going on a 1500 mile trip in ten days. Not feeling real comfortable with this truck now!! Please help!!!